A copy of a previously-unknown letter by Robert the Bruce imploring King Edward II to cease his persecution of the Scots in the build-up to Bannockburn has been discovered.

The letter, sent in 1310, shows Robert asserting his god-given authority as king of Scots and addressing Edward as his equal.

The letter has presented historians with fresh information about a pivotal time in the wars of Scottish independence as Scotland prepares to commemorate the 700th anniversary of the decisive Battle of Bannockburn next year.

Discovered by chance by a Glasgow historian, the document reveals how Robert made an appeal for peace when faced with an English army marching into the heart of Scotland, but on the understanding that Edward would recognise Scottish independence.

The document, which is thought to have been transcribed around the turn of the 16th century from the original letter, provides new insight into a time when Robert and Edward were experiencing different fortunes.

King Edward was growing increasingly unpopular with the nobles in his own court, according to historians, whilst Robert was slowly reclaiming power north of the border by winning the hearts and minds of the Scottish people.

Dauvit Broun, professor of Scottish history at the University of Glasgow, who made the discovery, said: "The letter reveals a couple of things: firstly, Bruce's tone is extremely conciliatory. He seems to be offering to do anything possible to establish peace.

"However, he is nonetheless plainly addressing Edward as one king to another. There is no doubt that the bottom line here is that Edward should recognise Robert as king of the Scots, and the Scots as separate from the English."

He added: "The writing of this letter should be seen as a bold move by Bruce, who had perhaps recognised that the tables were turning and he could stand his ground in the face of an advancing English army and open negotiations with the king. It allows us a clear sense of Bruce's terms, which were basically anything as long as Scottish independence is recognised."

By the time of the Battle of Bannockburn in 1314, Bruce had taken all the strongholds except Stirling and those near the English border. In the end, Bruce's move paid off as King Edward took his army south again to Berwick where he remained until July the next year. The next time he returned north, three years later, Edward was beaten at Bannockburn.